It is impossible to walk the Shikoku Pilgrimage without walking through a variety of tunnels.
I haven't sat down and calculated how many kilometers of tunnel you walk through, but it must be at least 20k and more probably double that, and in walking that is more than a day of distance.
It is possible to avoid some tunnels by taking a path up and over the mountain, but if you are carrying 20 kilos and/or the weather is bad then the tunnels makes sense....
Some of the tunnels are new, well lit, and with a decent sidewalk.
Some are old, dark, and narrow with just a painbted line separating you from the roaring traffic.
There are also quite a few traffic-free tunnels for pedestrians and cyclists.